How To Install Foam Board Insulation Under Siding

Jessica Stone
by Jessica Stone

Today, more than ever before, homes are being built with foam board insulation underneath the siding and on top of the outer sheathing. This detail is being used in both new constructions and renovation projects where contractors and DIYers are removing the original siding and installing a thin foam board underneath.

Installing foam board insulation underneath siding comes with a number of benefits including air leak prevention, improved moisture control and enhanced thermal protection. Installation is as simple as measuring the walls of your home, setting the panels, securing with nails and sealing up any seams between the panels.

We’ve outlined all the details of the various benefits and provided you with the necessary steps to properly install foam board insulation under your siding.

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Enhanced Thermal Performance

Installing exterior rigid foam board insulation on the exterior of a building can greatly improve the overall thermal performance. More specifically, it adds to the overall R value, which can be relatively substantial depending on the type and thickness of the insulation that is installed.

Quick Tip: The “R-value” is the building industry’s term that refers to thermal resistance “per unit area.” It involves the insulation’s ability to effectively resist the flow of heat. An R value can be established for a material (e.g. polyethylene foam), or a combination of materials (e.g. a specific wall, attic or your entire home).

Foam board insulation has R-values that range from 3.6 to 8.0 and has much better insulation per inch than materials such as fiberglass batts or plywood. This fact also helps to prevent mold or rot from occurring to the walls or framing of your home during damp temperatures.

Prevents Air Leaks

By far the biggest advantage, aside from an increase in R value, is the improved air filtration from a correctly installed layer of foam board insulation. The reduction in air filtration adds to the overall performance of the insulation behind the siding.

When foam board insulation is sealed using the correct techniques and an appropriate adhesive, it serves as an excellent air barrier. As opposed to house wrap, rigid foam board is able to both prevent air from entering your home and keep air from exiting it.

Moisture Control

Arguably the biggest draw for installing foam board insulation is its ability to control moisture. By affixing rigid foam board beneath your siding, it causes the dew point location to move outside of the wall assembly, protects from rain or other water from entering the siding and effectively reduces the risk of condensation forming inside the wall cavity.

Foam board insulation will also keep the siding, which is known to leak, from interacting with your home’s outer sheathing. It does this by creating another drainage plane that protects the sheathing and framing from possible damage caused by water infiltration.

Additionally, it warms the home’s framing or interior sheathing to prevent moisture from accumulating due to heated interior air during winter months.

Foam Board Insulation Disadvantages

Just like anything, foam bard insulation comes with its own set of downsides. The following are some of the disadvantages associated with rigid foam board insulation:

  • More expensive. Installing a layer of foam board insulation on top of OSB sheathing or plywood will increase the overall price of the project. However, this is just a short-term cost that will end up paying for itself with the reduction in utility bills for your home. It also may prevent the need to have any repairs or replacements done because of rotting to framing or walls.
  • Less structural strength than OSB sheathing or plywood. This fact does not apply if the foam board insulation is installed on top of wood sheathing. However, if you are using rigid foam board instead of wood sheathing, it will need additional fortification to prevent racking.
  • Must be installed properly. In order to create a weather-resistant barrier on your home and reduce air leaks, rigid foam must be installed correctly. There isn’t any specialized equipment required to install but you should follow particular seam-sealing procedures so that it’s up to code.

How to Install Foam Board Insulation Under Siding

Now that you have a better understanding of all the advantages and disadvantages that are associated with foam board insulation, the following are all the steps you need to take in order to properly install it underneath your siding.

Materials Needed:

  • Tape measure
  • XPS (or other type) of foam board insulation
  • Hammer
  • Utility knife
  • Reciprocal saw
  • Sealing Tape

Step 1: Measure the walls.

Using a tape measure, take measurements from the top to the bottom of your walls. Foam board insulation is available in either 4-by-8-foot or 4-by-9-foot sheets. You want to make sure that the rigid foam covers all the way from the home’s roof line to the base of the wall.

In most cases, ¾- to 1-inch thick XPS or extruded polystyrene is used, but consult your local building code for exact recommendations. Although there are other types, XPS is the most commonly used exterior foam board on the market. Check with your local hardware store to inquire about any regional variations or particular suggestions.

Step 2: Set the panels on the wall.

Beginning at the end of the house, place the panels on the wall, aligning the edges with the corner and base of the wall. Nail them in place to studs using galvanized nails with a 3/8-inch head. Make sure the nails are long enough to penetrate all the way through the foam and about ¾ inch into the wall.

Use a hammer to drive the nails through the foam board insulation into the studs. The nails should be spaced along each stud, 24 inches apart. Avoid overdriving and dimpling the foam; ensure that the nail heads are flush with the rigid foam.

Step 3: Cover an entire wall with panels.

Place the panels snugly together. Depending on the particular style, yours may have tongue and groove edges for easy fastening. Use a reciprocal saw to cut any necessary openings for windows and doors and trim around any small exterior protrusions using a utility knife.

Step 4: Seal the panel seams.

This step is important to ensure that the foam board insulation works as expected. Once the entire home is covered in the rigid foam, seal all of the seams between panels, door and window openings and wall corners. To seal properly, you’ll want to use specific tape that is supplied by the manufacturer of your foam board insulation.

Thoroughly tape all of the seams between panels vertically and make sure to close up any gaps that exist on the bottom and top edges and along windows and door frames.

Step 5: Install an additional moisture barrier.

For added moisture protection, wrap the entire house in polystyrene wrap, or some other type of moisture barrier.

Step 6: Mount the siding.

Once the foam board insulation and moisture barrier are installed, you can start mounting your siding. Installing the siding is as simple as nailing it through the foam board insulation into the framing of the house. Make sure that you use nails that are long enough to pierce through the siding, rigid foam and into the framing.

Installing House Wrap with Foam Board Insulation

One of the most common questions posed by homeowners is if house wrap can be installed along with foam board insulation. In short, yes, they can both be installed together and, when combined, they offer substantial insulation from outside weather and wind.

The other question that is often presented by both homeowners and contractors is, which product should go first? Should foam board insulation be installed underneath or on top of house wrap? Though some experts claim that it can be installed either way, there are others who assert that the foam board should go on top of the house wrap.

However, you can count on the advice of your particular contractor to make the final decision regarding your home. Some builders claim that homes with a “picture frame” window construction –with windows framed by strapping lumber before being installed– should have foam board insulation first, and the house wrap on top.

Either way you choose, your home will be effectively protected against air leaks and infiltration.

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Related Questions

Does foam board need a vapor barrier?

In most cases, rigid foam board meets all the requirements to serve as its own vapor barrier. However, when it is installed between studs, you want to tape or foam-seal all the areas where the foam meets timber. These gaps can enable vapor flow.

Should I install the soffit or siding first?

For best results, most experts recommend installing the soffit before the siding.

What type of wood goes underneath siding?

The wood under residential siding, or exterior sheathing, provides structural support, and adds waterproofing and insulation to the framing. It is also the surface that foam board insulation and the siding will be applied to. When it comes to the wood used for sheathing, the two options are OSB or plywood. Most builders prefer OSB, as it costs less, is more durable, and repels water better than traditional plywood.For more relevant insulation how-to’s, check out: “ How Much to Spray a 30×50 Metal Building with Closed Cell Foam?” and “ How To Insulate Exposed Roof Trusses

Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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